With tightening environmental regulations, there is an increasing need for real-time measurement of particle emissions. In particular, the need for measurement is present in the development of filtering methods, in the research of various combustion processes, as well as in monitoring processes for actual emissions. For classifying the mobility size distribution of aerosol particles, real-time aerosol measurements apply various analyzers which measure the electric mobility of particles, such as differential mobility analyzers (DMA).
FIG. 1 shows, in a simplified diagram, a mobility analyzer 10 according to prior art. The mobility analyzer 10 consists of an electrically conductive, preferably cylindrical frame part 12, which is coupled to a first constant potential, typically the ground plane, and thereby acts as an outer electrode. The ends of the frame part 12 are provided with cover and bottom plates 17a and 17b. An inner electrode 13 is placed centrally in the frame part 12 and coupled via a power supply 15 to a second constant potential. The power supply 15 is used to produce an electric field between the frame part 12 of the device and the inner electrode 13.
The cover and bottom plates 17a and 17b of the analyzer 10 are equipped with the necessary ducts to implement inlet and outlet pipes 18, 14 and 16 as well as the couplings required by the inner electrode 13.
So that particles could be separated on the basis of their electric mobility by means of the electric field, the aerosol particles to be introduced into the analyzer must have an electric charge before the analyzer 10. For this reason, the aerosol particles are typically charged by a separate charger (not shown in FIG. 1) before the analyzer 10. The flow 11 coming from the charger is led via the inlet pipe 18 to the analyzer. The flow 11 passes through the analyzer 10, primarily exiting via the outlet pipe 16.
When entering the electric field between the frame part of the analyzer 10 and the inner electrode 13, the charged aerosol particles in the flow 11 are drawn, depending on the sign of the charge, either towards the frame part 12 or towards the inner electrode 13. Typically, mobility analyzers are implemented in such a way that interesting aerosol particles are drawn towards the inner electrode 13.
The rate at which the aerosol particles drift towards the electrodes depends on the electric mobility of the particles, which is dependent, in a known manner, on e.g. the mass and charge of the particle.
Particles having greater electric mobility move faster towards the electrode determined by their charge, typically the inner electrode 13. As a result of this, the particles with greater electric mobility hit the inner electrode 13 sooner than particles with smaller electric mobility. As the flow 11 passes towards the bottom plate 17b of the analyzer 10, particles with greater electric mobility hit closer to the end of the inner electrode 13 on the side of the cover plate 17a, and particles with smaller mobility hit closer to the end of the inner electrode 13 on the side of the bottom plate 17b. Those particles whose electric mobility is so small that they do not reach the inner electrode 13 when moving with the flow 11 through the analyzer, are discharged via the outlet pipe 16 from the analyzer.
Depending on the solution to be used, the above-presented mobility analyzer 10 can be implemented by a number of different ways. In its simplest form, the solution does not apply the outlet pipe 14 placed inside the electrode 13 at all, wherein the complete flow 11 coming into the analyzer 10 is discharged through orifices in the bottom plate 17b into the outlet pipe 16. Thus, it is possible to use the analyzer to remove from the flow 11 the particles which have greater electric mobility than a certain value and which are deposited on the inner electrode 13.
DMA analyzers are typically implemented in such a way that the inner electrode 13 is provided with a narrow slit 19 which is coupled to the second outlet pipe 14. Thus, particles falling into the slit 19 are absorbed into the second outlet pipe 14. If the analyzer 10 is constructed in such a way that the flow 11 to be measured is introduced into the analyzer 10 at a certain distance from the inner electrode 13, the voltage difference between the frame part 12 of the analyzer 10 and the inner electrode 13 can be adjusted to determine the mobility range of the particles falling into the slit 19. Thus the DMA analyzer can be used to separate particles falling into a certain mobility range from the flow 11 under analysis, by guiding them into the second outlet pipe 14, wherein particles with greater electric mobility adhere to the inner electrode 13 before the slit 19 and particles with smaller mobility are discharged with the flow along the outlet pipe 16.
Furthermore, it is known to implement the mobility analyzer 10 as a multi-channel differential mobility analyzer, in which the inner electrode 13 of the mobility analyzer 10 is equipped with several detection surfaces which indicate the number of particles hitting them, for example on the basis of the charges transferred by the particles hitting them. The detection surfaces are preferably placed onto the surface of the inner electrode 13 in such a way that each of them collects particles falling into a certain mobility range. By monitoring the signals given by these detection surfaces, it is possible to measure, in real time, the electric mobility of particles in the flow 11 under analysis and, on the basis of this, to compute the size distribution based on the electric mobility diameter of the particles.
The mobility analyzers are most precise for particles with a small mass, because the electric mobility of the particles is inversely proportional to the mass of the particle; that is, the greater the mass of the particle, the smaller the mobility of the particle. In typical particle measurement environments, the aim is to measure particles whose diameters vary from some tens of nanometers to tens of micrometers. In this range, there are typically differences of several orders in the mobility of the particles, wherein it is extremely difficult to measure the whole range simultaneously by one device with a sufficiently high precision. Electric mobility analyzers are primarily used in fine particle analyses, in which the mobility analyzers are most precise, due to the high mobility of the fine particles.
For measuring primarily larger particles, classification methods based on the aerodynamic diameter of the particles are normally used, such as impactors. The electrical low pressure impactor (ELPI) developed by Dekati Ltd provides a solution for real-time measurement of aerosol particles.
FIG. 2 shows a cross-sectional view of two upper stages 21a and 21b of an electrical low pressure impactor 20, and chambers 29a and 29b related to them. An air flow 11 to be analyzed is sucked by means of an underpressure produced by a pump (not shown in FIG. 2) into the electrical low pressure impactor 20 and into the first chamber 29a of the impactor. Each stage comprises a nozzle part 22a, 22b, equipped with orifices, through which the air flow 11 carrying particles flows. Collection surfaces 23a, 23b are placed behind the nozzle parts 22a, 22b. Each collection surface 23a, 23b is equipped with at least one outlet 25, through which the flow 11 is allowed to flow to the next chamber or out of the impactor. Insulators 24a, 24b, 24c placed between the stages 20a, 20b insulate the different stages 20a, 20b from each other and the first stage of the impactor from the cover part 26.
The direction of the air flow 11 flowing from the orifices of the nozzle part 22a, 22b is abruptly changed when it meets the collection surface 23a, 23b. The particles carried by the flow 11 and having a sufficiently large aerodynamic particle size cannot follow the abrupt change in the direction of the flow, but they hit the collection surface 23a, 23b, being deposited on the same. When hitting the collection surface 23a, 23b, the charged particles cause a change in the charge level of the collection surface 23a, 23b. As the collection surface 23a, 23b is electrically coupled to said impactor stage 20a, 20 b which is, via an electrical coupling 27a, 27b further connected to a control u charge level of the collection surface 23a, 23b is indicated as an electric current which can be detected by sensitive current meters placed in the control unit 28.
The above-described method makes it possible to classify the particles selectively according to the size. By selecting, in a known way, the number and size of orifices in the nozzle part 22a, 22b, the distance between the nozzle part 22a, 22b and the collection surface 23a, 23b, as well as the flow rate to be used, each impactor stage 20a, 20b can be dimensioned so that only particles larger than a given particle size are deposited on the collection surface at each stage. By dimensioning successive impactor stages in such a way that particles with different particle sizes are collected on different stages, the currents from the different impactor stages 20a, 20b measured by the control unit 28 can be used to determine, in real time, the size distribution of the particles in the flow 11 to be measured, on the basis of the aerodynamic diameter.
As a result of the operating principle of the impactor, the impactors are most sensitive to particles whose mechanical mobility is small, i.e. typically particles with a large aerodynamic diameter. On the other hand, it is difficult to measure small particles with the impactors, because, due to the high mechanical mobility of the particles, high flow rates and abrupt changes in the direction of flow are thus required to separate the particles from the flow.
A problem with the above-presented real-time particle measurements of prior art has been their inapplicability for measurements in a wide range of particle sizes. DMA analyzers are suitable for the measurement of particles with a small diameter and high electric mobility, and impactors are suitable for the measurement of particles with a very large aerodynamic diameter.
In certain applications, it is advantageous to carry out the measurement of the particle size distribution within a wide range of particle sizes. If the measurement is made with an electric mobility analyzer, the problem is the poor sensitivity of the device for large particles, and if an impactor is used, the problem is the detection of small particles with a sufficient precision. Attempts have been made to solve this problem with prior art devices by carrying out the measurement in two parts, wherein an electric mobility analyzer 10 is first used to measure the size distribution of small particles. After this, the flow obtained from the mobility analyzer 10 through the outlet pipe 16 is guided to a separate impactor 20 to determine the size distribution of larger particles.
A problem in the above-described solution of prior art lies in the joint operation of the two separate measuring devices. Joint measurements with other measuring devices are typically not taken into account in the design of the measuring devices, wherein it is difficult to implement the centralized control of the different measuring devices. Furthermore, a problem may be presented by particle losses in the transfer pipe system between the different measuring devices.